This invention relates to a variable reluctance sensor. More specifically, this invention relates to a reluctance sensor which enhances low output signals by reducing magnetic flux leakage.
It is desirable in many applications to determine the precise position or speed of various objects which linearly traverse a defined path or a rotational path. For example, the position or speed of numerous items in the automotive field such as transmission input and output shafts, crankshaft or tone wheels for anti-lock braking systems. A common type of sensor used in automotive components is a variable reluctance sensor which functions by sensing the change in reluctance from a ferrous target as the air gap between the sensor and the target changes. This is usually accomplished by cutting slots in the target and passing it by the sensor.
A variable reluctance sensor is used because it is rugged and is of relatively low cost. The variable reluctance sensor typically has a pole piece with a coil of wire. A magnet is located in the sensor and generates a magnetic field in the coil. The target object is a ferrous material such as steel or iron whose movement effects the flux within the coil generated by the magnet. For example, a target object may be a gear whose teeth are in proximity to the variable reluctance sensor. The reluctance is measured from the pole piece or magnet and is proportional to the distance from the target object. The magnet may either be in the front of the pole or the rear of the pole piece. Another configuration is having a stacked magnetic material forming a rectangular pole piece having alternating ferrous and magnetic material. The reluctance change on one end of the stack also changes the flux paths in the other end which creates the voltage change in the coil.
The output of the variable reluctance sensor depends on the rate of change of magnetic flux in the coil. The voltage generated by the coil is proportional to the number of turns in the coil. Given the air gap between the pole piece and the target object as well as magnetic paths through the pole piece and other sensor assembly, some of the magnetic flux leaks and does not affect the coil. Generally, it is desirable to have a higher voltage output for easier reading of the sensor output. Thus, the more leakage of magnetic field flux, the lower the voltage output. At low speeds of the target object, lower rate of change of flux is generated in the coil, which combined with inherent leaks, results in a lower voltage output from the sensor.
The present reluctance sensors may be enhanced in order to increase low voltage output. For example, added ferrous paths may be created by providing additional pole pieces near the target object. The additional pole pieces allow the capture of additional flux thus increasing overall voltage output. However, these modifications add cost and complexity to the system, nullifying the advantages of the variable reluctance sensor. Such corrections also require extra processing for the additional pole pieces which increase the complexity and cost of the device.
Thus, there exists a need for an increased output reluctance sensor. There is also a need for a reluctance sensor which does not require excessive processing components. There is a further need for a reluctance sensor which provides high output from target objects with low flux levels.
The present invention may be embodied in a variable reluctance sensor for determining the position of a moveable target object. The sensor has a magnet and a pole piece coupled to the magnet. A wire coil is located annularly around the pole piece. A second bucking magnet is annularly located around the coil which generates a flux field. A control unit is coupled to the wire coil which measures the sinusoidal signal generated in the coil from the movement of the target object.
The present invention may also be embodied in a method of increasing the voltage output from a variable reluctance sensor. The sensor has a pole piece, a coil located annularly around the pole piece, and a magnet. A bucking magnet is added around the coil to produce flux to cancel leaking flux from the sensor to increase coil voltage output.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.